Schottky structure employing central implants between junction barrier elements

ABSTRACT

The present disclosure relates to a Schottky diode having a drift layer and a Schottky layer. The drift layer is predominantly doped with a doping material of a first conductivity type and has a first surface associated with an active region. The Schottky layer is provided over the active region of the first surface to form a Schottky junction. A plurality of junction barrier elements are formed in the drift layer below the Schottky junction, and a plurality of central implants are also formed in the drift layer below the Schottky junction. In certain embodiments, at least one central implant is provided between each adjacent pair of junction barrier elements.

FIELD OF THE DISCLOSURE

The present disclosure relates to semiconductor devices, and inparticular to employing central implants between junction barrierelements along a Schottky interface.

BACKGROUND

A Schottky diode takes advantage of a metal-semiconductor junction,which provides a Schottky barrier and is created between a metal layerand a doped semiconductor layer. For a Schottky diode with an N-typesemiconductor layer, the metal layer acts as the anode, and the N-typesemiconductor layer acts as the cathode. In general, the Schottky diodeacts like a traditional p-n diode by readily passing current in theforward-biased direction and blocking current in the reverse-biaseddirection. The Schottky barrier provided at the metal-semiconductorjunction provides two unique advantages over p-n diodes. First, theSchottky barrier is associated with a lower barrier height, whichcorrelates to lower forward voltage drops. As such, a smaller forwardvoltage is required to turn on the device and allow current to flow in aforward-biased direction. Second, the Schottky barrier generally hasless capacitance than a comparable p-n diode. The lower capacitancetranslates to higher switching speeds than p-n diodes. Further, Schottkydiodes are majority carrier devices and do not exhibit minority carrierbehavior, which results in switching losses.

Unfortunately, Schottky diodes have traditionally suffered fromrelatively low reverse-biased voltage ratings and high reverse-biasedleakage currents. In recent years, Cree, Inc. of Durham, N.C., hasintroduced a series of Schottky diodes that are formed from siliconcarbide substrates and compatible epitaxial layers. These devices haveand continue to advance the state of the art by increasing thereverse-biased voltage ratings, lowering reverse-biased leakagecurrents, and increasing forward-biased current handling. However, thereremains a need to further improve Schottky device performance as well asreduce the cost of these devices.

SUMMARY

The present disclosure relates to a Schottky diode having a drift layerand a Schottky layer. The drift layer is predominantly doped with adoping material of a first conductivity type and has a first surfaceassociated with an active region. The Schottky layer is provided overthe active region of the first surface to form a Schottky junction. Aplurality of junction barrier elements are formed in the drift layerbelow the Schottky junction, and a plurality of central implants arealso formed in the drift layer below the Schottky junction. In certainembodiments, at least one central implant is provided between eachadjacent pair of junction barrier elements.

The Schottky layer may be formed from a low barrier height material,such as titanium, chromium, polysilicon, aluminum, or any other suitablematerial. An even lower barrier height material, such as tantalum, worksextremely well with a drift layer formed from silicon carbide. Thecentral implants and the junction barrier elements generally extend intothe drift layer to differing depths. In one embodiment, the centralimplants have a depth that is no greater than about one-half of thedepth of each of the junction barrier elements. In another embodiment,the junction barrier elements are at least four times deeper than eachof the plurality of central implants.

In certain embodiments, the first surface of the drift layer hasnumerous junction barrier element recesses formed in the active region,such that at least certain junction barrier elements of the plurality ofjunction barrier elements are doped regions that extend into the driftlayer about corresponding ones of the plurality of junction barrierelement recesses. The doped regions are doped with a doping material ofa second conductivity type, which is opposite the first conductivitytype.

Further, a buffer region may be provided in a top portion of the driftlayer. The buffer region is doped with the doping material of the firstconductivity type at a higher concentration than a remaining lowerportion of the drift layer, and both the central implants and thejunction barrier elements reside in the buffer region.

Those skilled in the art will appreciate the scope of the disclosure andrealize additional aspects thereof after reading the following detaileddescription in association with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thisspecification illustrate several aspects of the disclosure, and togetherwith the description serve to explain the principles of the disclosure.

FIG. 1 is a cross-sectional view of a Schottky diode according to afirst embodiment of the disclosure.

FIG. 2 is a top view of an active region of the Schottky diode, withoutthe Schottky layer and anode contact, according to one embodiment of thedisclosure.

FIG. 3 is a top view of an active region of the Schottky diode, withoutthe Schottky layer and anode contact, according to another embodiment ofthe disclosure.

FIG. 4 is a partial cross-sectional view of a Schottky diode accordingto one embodiment of the disclosure.

FIG. 5 is a cross-sectional view of a Schottky diode according to asecond embodiment of the disclosure.

FIG. 6 is a cross-sectional view of a Schottky diode according to athird embodiment of the disclosure.

FIGS. 7A, 7B, and 7C are graphs that illustrate the relative electricalfield distributions along the Schottky interfaces of three differentjunction barrier element configurations.

FIG. 8 is a cross-sectional view of a Schottky diode according to afourth embodiment of the disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the disclosure andillustrate the best mode of practicing the disclosure. Upon reading thefollowing description in light of the accompanying drawings, thoseskilled in the art will understand the concepts of the disclosure andwill recognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The present disclosure relates to a Schottky diode having a drift layerand a Schottky layer. The drift layer is predominantly doped with adoping material of a first conductivity type and has a first surfaceassociated with an active region. The Schottky layer is provided overthe active region of the first surface to form a Schottky junction. Aplurality of junction barrier elements are formed in the drift layerbelow the Schottky junction, and a plurality of central implants arealso formed in the drift layer below the Schottky junction. In certainembodiments, at least one central implant is provided between eachadjacent pair of junction barrier elements.

The Schottky layer may be formed from a low barrier height material,such as titanium, chromium, polysilicon, and aluminum. An even lowerbarrier height material, such as tantalum, works extremely well with adrift layer formed from silicon carbide. The central implants and thejunction barrier elements generally extend into the drift layer todiffering depths. In one embodiment, the central implants have a depththat is no greater than about one-half of the depth of each of thejunction barrier elements. In another embodiment, the junction barrierelements are at least four times deeper than each of the plurality ofcentral implants.

In certain embodiments, the first surface of the drift layer hasnumerous junction barrier element recesses formed in the active region,such that at least certain junction barrier elements of the plurality ofjunction barrier elements are doped regions that extend into the driftlayer about corresponding ones of the plurality of junction barrierelement recesses. The doped regions are doped with a doping material ofa second conductivity type, which is opposite the first conductivitytype.

Further, a buffer region may be provided in a top portion of the driftlayer. The buffer region is doped with the doping material of the firstconductivity type at a higher concentration than a remaining lowerportion of the drift layer, and both the central implants and thejunction barrier elements reside in the buffer region.

An exemplary Schottky diode 10 is provided in association with FIG. 1.Notably, the embodiments described herein reference varioussemiconductor layers or elements therein as being doped with an N-typeor P-type doping material. Being doped with an N-type or P-type materialindicates that the layer or element has either an N-type or P-typeconductivity, respectively. N-type material has a majority equilibriumconcentration of negatively charged electrons, and P-type material has amajority equilibrium concentration of positively charged holes. Thedoping concentrations for the various layers or elements may be definedas being lightly, normally, or heavily doped. These terms are relativeterms intended to relate doping concentrations for one layer or elementto those for another layer or element.

Further, the following description focuses on an N-type substrate anddrift layer being used in a Schottky diode; however, the conceptsprovided herein are equally applicable to Schottky diodes with P-typesubstrates and drift layers. As such, the doping charge for each layeror element in the disclosed embodiments may be reversed to createSchottky diodes with P-type substrates and drift layers. Further, any ofthe layers described herein may be formed from one or more epitaxiallayers using any available technique, and additional layers that are notdescribed may be added between those described herein withoutnecessarily departing from the concepts of the disclosure.

As illustrated, the Schottky diode 10 is formed on a substrate 12 andhas an active region 14 that resides within an edge termination region16 that may, but does not need to, completely or substantially surroundthe active region 14. Along the bottom side of the substrate 12, acathode contact 18 is formed and may extend below both the active region14 and the edge termination region 16. A cathode ohmic layer 20 may beprovided between the substrate 12 and the cathode contact 18 tofacilitate a low impedance coupling therebetween. A drift layer 22extends along the top side of the substrate 12. The drift layer 22, thecathode contact 18, and the cathode ohmic layer 20 may extend along boththe active region 14 and the edge termination region 16.

In the active region 14, a Schottky layer 24 resides over the topsurface of the drift layer 22, and an anode contact 26 resides over theSchottky layer 24. A barrier layer 28 may be provided between theSchottky layer 24 and the anode contact 26 to prevent materials from oneof the Schottky layer 24 and the anode contact 26 from diffusing intothe other. Notably, the active region 14 substantially corresponds tothe region where the Schottky layer 24 of the Schottky diode 10 residesover the drift layer 22. For purposes of illustration only, assume thesubstrate 12 and the drift layer 22 are silicon carbide (SiC). Othermaterials for these and other layers are discussed further below.

In the illustrated embodiment, the substrate 12 is heavily doped and thedrift layer 22 is relatively lightly doped with an N-type material. Thedrift layer 22 may be substantially uniformly doped or doped in a gradedfashion. For example, doping concentrations of the drift layer 22 maytransition from being relatively more heavily doped near the substrate12 to being more lightly doped near the top surface of the drift layer22 that is proximate the Schottky layer 24, and vice versa Dopingdetails for various exemplary embodiments are provided further below.

Beneath the Schottky layer 24, a plurality of junction-barrier (JB)elements 30 are provided along the top surface of the drift layer 22.Further, unique elements, which are referred to as central implants 32,are also provided along the top surface of the drift layer 22. Asillustrated, a central implant 32 is provided between each pair ofadjacent JB elements 30. Typically, the cross-section of the centralimplants 32 is substantially smaller than the cross-section of the JBelements 30, as depicted in FIG. 1. Heavily doping select regions in thedrift layer 22 with P-type material forms both the JB elements 30 andthe central implants 32. As such, each JB element 30 and central implant32 extends from the top surface of the drift layer 22 into the driftlayer 22. Together, the JB elements 30 and the central implants 32 forma JB array.

The JB elements 30 and central implants 32 may take on various shapesand be laid out in different configurations, as illustrated in FIGS. 2and 3. As illustrated in FIG. 2, each JB element 30 and central implant32 is a single, long, elongated stripe that extends substantially acrossthe active region 14, wherein the JB array is a plurality of alternatingJB elements 30 and central implants 32. In FIG. 3, each JB element 30and central implant 32 is a short, elongated dash wherein the JB arrayhas parallel rows of multiple dashes that are linearly aligned to extendacross the active region 14. Ohmic elements 30A may be provided alongthe top surface of the drift layer 22 within the active region 14. TheOhmic elements 30A help increase the ability of the active region 14 tohandle current and voltage surges. The Ohmic elements 30A are metalelements that are used to make a good Ohmic contact with the JB elements30. The Ohmic elements 30A may be formed by existing photo lift-off ormetal wet etch techniques.

As described further below, the elongated stripes and the dashes mayhave substantially the same or substantially different dopingconcentrations. Other shapes and configurations of the JB elements 30and central implants for the JB array that is formed therefrom will beappreciated by those skilled in the art after reading the disclosureprovide herein.

With continued reference to FIG. 1, the edge termination region 16 thatis formed in a portion of the drift layer 22 and completely, or at leastsubstantially, surrounds the active region 14. A guard ring-typetermination is provided as an example. Other termination techniques,such as Bevel, Junction Termination Extension (JTE), and the like couldbe applied as alternatives to the guard ring termination. A recess well34 is formed by lightly doping a portion of the drift layer 22 thatresides below the surface of the drift layer 22 with a P-type material.As such, the recess well 34 is a lightly doped P-type region within thedrift layer 22. The recess well 34 may, but need not, extend to orpartially into the outer most JB elements 30. Along the surface of thedrift layer 22 and within the recess well 34, a plurality of concentricguard rings 36 are formed. The guard rings 36 are formed by heavilydoping the corresponding portions of the recess well 34 with a P-typedoping material. In select embodiments, the guard rings are spaced apartfrom one another and extend into the recess well 34 from the top surfaceof the drift layer 22.

In addition to the guard rings 36 that reside in the recess well 34, amesa guard ring (not shown) may be provided around the outer peripheryof the active region 14. The mesa guard ring is also formed by heavilydoping a portion of the drift layer 22 about active region 14, such thatthe mesa guard ring is formed about the periphery of the active region14 and extends into the drift layer 22. The recess well 34, the guardrings 36, and the mesa guard ring may be of any shape and will generallycorrespond to the shape of the periphery of the active region 14, whichis rectangular in the illustrated embodiments. Each of these threeelements may provide a continuous or broken (i.e. dashed, dotted, or thelike) loop about the active region 14.

In one embodiment, FIG. 4 provides an enlarged view of a portion of theactive region 14 and is used to help identify the various p-n junctionsthat come into play during operation of the Schottky diode 10. For thisembodiment, assume the JB elements 30 and central implants 32 areelongated stripes (as illustrated in FIG. 2). With the presence of theJB elements 30 and the central implants 32, there are at least two typesof junctions about the active region 14. The first junction type isreferred to as a Schottky junction J1, and is any metal-semiconductor(m-s) junction between the Schottky layer 24 and those portions of thetop surface of the drift layer 22 that do not have a JB element 30 orcentral implant 32. In other words, the Schottky junction J1 is ajunction between the Schottky layer 24 and the those portions of the topsurface of the drift layer that are between a JB element 30 and acentral implant 32. The second junction type is referred to as a JBjunction J2, and is any p-n junction between the drift layer and eithera JB element 30 or central implant 32 that is formed in the drift layer22.

As the Schottky diode 10 is forward-biased, the Schottky junctions J1turn on before the JB junctions J2 turn on. At relatively low forwardvoltages, current transport in the Schottky diode 10 is dominated bymajority carriers (electrons) injected across the Schottky junction J1.As such, the Schottky diode 10 acts like a traditional Schottky diode.In this configuration, there is little or no minority carrier injection,and thus no minority charge. As a result the Schottky diode 10 iscapable of fast switching speeds at normal operating voltages.

When the Schottky diode 10 is reverse-biased, depletion regions thatform adjacent the JB junctions J2 expand to block reverse currentthrough the Schottky diode 10. As a result, the expanded depletionregions function to both protect the Schottky junction J1 and limitreverse leakage current in the Schottky diode 10. With the JB elements30 and the central implants 32, the Schottky diode 10 behaves like a PINdiode.

In almost any Schottky diode design, there are the basic needs for 1) alow forward-biased resistance, which corresponds to a low forward-biasedvoltage drop, 2) a low reverse-biased leakage current, and 3) a costeffective design. With regard to cost, the size of the die on which theSchottky diode is fabricated is a significant contributor to the overallcost. As such, there is constant pressure to maintain or increase theperformance of the Schottky diode while reducing the size of the die onwhich the Schottky diode is fabricated.

To decrease the die size while at least maintaining the forward-biasedcurrent rating for the Schottky diode, the forward-biased resistancemust be reduced. One way to reduce the forward-biased resistance is toincrease the doping on the drift layer 22. Unfortunately, increasing thedoping in the drift layer 22 has the adverse effect of increasing thereverse-biased leakage current. Thus, there is a need to reduce thereverse-biased leakage current without adversely affecting theforward-biased resistance and voltage drop.

To reduce the reverse-biased leakage current, the electric field in theSchottky interface, which is generally the interface between the driftlayer 22 and the Schottky layer 24, should be reduced. The electricfield in the Schottky interface is roughly proportional to thereverse-biased leakage current. It has been discovered that the use ofthe central implants 32 between the JB elements 30 in a JB array asillustrated above significantly reduces the electric field in theSchottky interface relative to a JB array that does not employ centralimplants 32. Thus, the drift layer 22 may be more highly doped to reducethe forward-biased resistance while having little or no impact on thereverse-biased leakage current when the central implants 32 are employedin the JB array, as illustrated in FIG. 1. As described below inassociation with FIGS. 5 and 6, additional steps may be taken to evenfurther reduce the electric field in the Schottky interface.

With reference to FIG. 5, the drift layer 22 and the Schottky layer 24of Schottky diode 10 are illustrated according to an alternativeembodiment. As illustrated, each of the JB elements 30 includes a JBelement recess 30R, which is etched into the top surface of the driftlayer 22. By etching recesses into the drift layer 22, the respective JBelements 30 may be formed more deeply into the drift layer 22. This isparticularly beneficial for SiC devices, wherein the JB element recesses30R may be formed prior to the selective doping used to form the JBelements 30. The JB element recesses 30R allow deeper doping into thedrift layer 22. When describing the width of a particular JB elementrecess 30R, the width refers to the narrower lateral dimension of arecess having a width, length, and depth. In one embodiment, the depthof any recess is at least 0.8 microns, and the width of any recess is atleast 0.8 microns. In another embodiment, the depth of is recesses areat least 2 microns, and the width of any recess is at least 2 microns.Employing the JB element recesses 30R in the JB elements 30 along withthe central implants 32 appears to provide a greater electric fieldreduction in the Schottky interface than simply using the JB elements 30and central implants 32.

The embodiment illustrated in FIG. 6 has proven to provide an evengreater electric field reduction the Schottky interface. In particular,the Schottky diode 10 is shown with deeply implanted JB elements, whichare referred to as deep JB elements 30′. The deep JB elements 30′ mayinclude JB element recesses 30R, as depicted. Central implants 32 may beprovided between the deep JB elements 30′.

Notably, the drift layer 22 is shown to include a buffer region 38,which is formed along the top portion of the drift layer 22. The bufferregion 38 may be more heavily doped than the remainder, or lowerportion, of the drift layer 22. The deep JB elements 30′ and the centralimplants 32 are formed in the buffer region 38. In this embodiment, thedeep JB elements 30′ extend substantially through the buffer region 38and to, or at least near, the bottom of the buffer region 38.

FIGS. 7A, 7B, and 7C are graphs that plot the relative electric fielddistributions along the Schottky interface 42 for three different JBarray configurations. FIG. 7A plots the relative electric field alongthe Schottky interface 42 for a JB array that only includes JB elements30. No central implants 32 are provided between the JB elements 30.Along the top surface of each of the two illustrated JB elements 30, theelectric field is relatively low. However, the electric field quicklyrises to a relatively high level at the midpoint between the two JBelements 30.

FIG. 7B plots the relative electric field along the Schottky interface42 for a JB array that includes JB elements 30 and interspersed centralimplants 32, as provide in FIG. 1. Along the top surface of each of thetwo illustrated JB elements 30 and the central implant 32 providedtherebetween, the electric field is relatively low. However, theelectric field rises to a relatively low level at the two midpointsbetween the central implant 32 and each of the JB elements 30.

FIG. 7C plots the relative electric field along the Schottky interface42 for a JB array that includes deep JB elements 30′ and interspersedcentral implants 32, as provide in FIG. 6. Along the top surface of eachof the two deep JB elements 30′ and the central implant 32 providedtherebetween, the electric field is relatively low. The electric fieldonly rises to a very low level at the two midpoints between the centralimplant 32 and each of the deep JB elements 30′. These plots clearlydepict the significant advantage in using central implants 32 along withJB elements 30 (including deep JB elements 30′) in a JB array and theadditional benefit in using deep JB elements 30′ with JB elementrecesses 30R.

While the above embodiments are directed to Schottky diodes 10, all ofthe contemplated structures and designs are equally applicable to othersemiconductor devices. Exemplary devices that may benefit from thecontemplated structures and designs include all types of field effecttransistors (FETs), insulated gate bipolar transistors (IGBTs), and gateturn-off thyristors (GTOs).

Another characteristic that affects both forward and reverse current andvoltage characteristics of the Schottky diode 10 is the barrier heightassociated with the Schottky junction J1 (FIG. 4), which again, is themetal-semiconductor junction between the metal Schottky layer 24 and thesemiconductor drift layer 22. When a metal layer, such as the Schottkylayer 24, is in close proximity with a semiconductor layer, such as thedrift layer 22, a native potential barrier develops between the twolayers. The barrier height associated with the Schottky junction J1corresponds to the native potential barrier. Absent application of anexternal voltage, this native potential barrier prevents most chargecarriers, either electrons or holes, from moving from one layer toanother the other. When an external voltage is applied, the nativepotential barrier from the semiconductor layer's perspective willeffectively increase or decrease. Notably, the potential barrier fromthe metal layer's perspective will not change when the external voltageis applied.

When a Schottky diode 10 with an N-type drift layer 22 is forwardbiased, application of a positive voltage at the Schottky layer 24effectively reduces the native potential barrier and causes electrons toflow from the semiconductor across the metal-semiconductor junction. Themagnitude of the native potential barrier, and thus barrier height,bears on the amount of voltage necessary to overcome the nativepotential barrier and cause the electrons to flow from the semiconductorlayer to the metal layer. In effect, the potential barrier is reducedwhen the Schottky diode is forward biased. When the Schottky diode 10 isreverse biased, the potential barrier is greatly increased and functionsto block the flow of electrons.

The material used to form the Schottky layer 24 largely dictates thebarrier height associated with the Schottky junction J1. In manyapplications, a low barrier height is preferred. A lower barrier heightallows one of the following. First, a lower barrier height device with asmaller active region 14 can be developed to have the same forward turnon and operating current and voltage ratings as a device having a largeractive region 14 and a higher barrier height.

In other words, the lower barrier height device with a smaller activeregion 14 can support the same forward voltage at a given current as adevice that has a higher barrier height and a larger active region 14.Alternatively, a lower barrier height device may have lower forward turnon and operating voltages while handling the same or similar currents asa higher barrier height device when both devices have active regions 14of the same size. Lower barrier heights also lower the forward biasedon-resistances of the devices, which help make the devices moreefficient and generate less heat, which can be destructive to thedevice.

Exemplary metals (including alloys) that are associated with low barrierheights in Schottky applications that employ a SiC drift layer 22include, but are not limited to, tantalum (Ta), polysilicon, titanium(Ti), chromium (Cr), and aluminum (Al), where tantalum is associatedwith the lowest barrier height of the group. The metals are defined aslow barrier height cable metals. While the barrier height is a functionof the metal used for the Schottky layer 24, the material used for thedrift layer 22, and perhaps the extent of doping in the drift layer 22,exemplary barrier heights that may be achieved with certain embodimentsare less than 1.2 election volts (eV), less than 1.1 eV, less than 1.0eV, less than 0.9 eV, and about 0.8 eV.

In select embodiments, the substrate 12 is formed from an N-doped,single crystal, 4H SiC material. The substrate 12 may have variouscrystalline polytypes, such as 2H, 4H, 6H, 3C and the like. Thesubstrate 12 may also be formed from other material systems, such asgallium nitride (GaN), gallium arsenide (GaAs), silicon (Si), germanium(Ge), SiGe, and the like. The resistivity of the N-doped, SiC substrate12 is between about 10 milli-ohm·cm and 30 milli-ohm·cm in oneembodiment. The initial substrate 12 may have a thickness between about200 microns and 500 microns. Once the epitaxial structure above thesubstrate is formed, the back side of the substrate 12 may be thinnedprior to forming the cathode contact 18.

The drift layer 22 may be grown over the substrate 12 and doped in situ,wherein the drift layer 22 is doped as it is grown with an N-type dopingmaterial. Notably, one or more buffer layers (not shown) may be formedon the substrate 12 prior to forming the drift layer 22. The bufferlayer may be used as a nucleation layer and be relatively heavily dopedwith an N-type doping material. The buffer layer may range from 0.5 to 5microns in certain embodiments.

The drift layer 22 may be relatively uniformly doped throughout or mayemploy graded doping throughout all or a portion thereof. For auniformly doped drift layer 22, the doping concentration may be betweenabout 2×10¹⁵ cm⁻³ and 2×10¹⁶ cm⁻³ in one embodiment. If the bufferregion 38 (FIG. 6) is provided, the buffer region is formed by dopingthe upper portion of the drift layer 22 at a higher level than thebottom portion or portions of the drift layer 22. While differentperformance metrics may generally necessitate different doping levelsand thicknesses, the following Table A provides exemplary doping levelsand thicknesses for the overall drift layer 22 and the buffer region 38,which resides in the top portion of the drift layer 22, for a Schottkydiode 10 that has a 50 ampere forward-biased current rating at differentreverse-biased breakdown voltages.

TABLE A Buffer Drift Layer Drift Layer Region Breakdown Doping N_(d)Thickness Buffer Region Thickness Voltage (cm⁻³) (microns) Doping (cm⁻³)(microns)  650 V 8e15-1.5e16 4-8 >2 × N_(d)* 0.5-2 1200 V 7e15-1e16 9-14 >2 × N_(d)* 0.5-2 1700 V 5e15-8e15 12-19 >3 × N_(d)* 0.5-2  10 kV2e14-5e14 >100 >2e16* 0.5-2 *and less than 1e18

The recess well 34 may be formed by lightly implanting select portionsof the drift layer 22 with a P-type material. Similarly, the JB elements30 (or deep JB elements 30′), the central implants 32, and the guardrings 36 may be formed by implanting the corresponding portions of thetop surface of the drift layer 22 with a P-type material. The JBelements 30, the central implants 32, and the guard rings 36 arerelatively heavily doped and may be formed at the same time using thesame ion implantation process. The JB element recesses 30R may be formedprior to implantation to aid in achieving a deeper and more uniformdoping concentration for the corresponding JB element 30 (or deep JBelements 30′). In one embodiment, the JB elements 30, central implants32, and the guard rings 36 are all doped at substantially the sameconcentrations. Typically, the JB elements 30, central implants 32, andthe guard rings 36 are all doped at concentrations of about 1×10¹⁸ cm⁻³or greater. In other embodiments, these elements may be doped atdifferent concentrations using the same or different ion implantationprocess, for example, when the JB array includes different shapes orsizes or where the different JB elements 30 have different depths.

The depth and spacing between adjacent JB elements 30, the centralimplants 32, and the guard rings 36 may vary based on desired devicecharacteristics. In one embodiment, the depth of the central implants 32may generally range between about 0.2 and 0.6 microns. The width of thecentral implants 32 may generally range between about 0.9 and 1.6microns. The depth of the JB elements 30 may generally range betweenabout 0.5 and 5.0 microns. The width of the JB elements 30 may generallyrange between about 1.5 and 2.0 microns. Each central implant 32 may bespaced apart from an adjacent JB element 30 between about 1.5 to 2.5microns. In certain embodiments, the depth of a central implant 32 is nogreater than about one-half the depth of the JB element 30 (includingdeep JB elements 30′). In certain embodiments, the depth of the JBelements 30 is about three, four, five or more times the depth of thecentral implants 32.

For embodiments like those illustrated in FIGS. 5 and 6 that employ JBelement recesses 30R, the JB elements 30 are typically more easilyformed deeper into the drift layer 22. For a drift layer 22 that isformed from SiC, the depth of the respective recesses may be betweenabout 0.5 and 2 microns and have widths of between about 0.8 and 2microns.

During fabrication, a thermal oxide layer (not shown) may be formed overthe top surface of the drift layer 22. For a SiC drift layer 22, theoxide is silicon dioxide (SiO₂). The thermal oxide layer may act as apassivation layer that aids in the protection or performance of thedrift layer 22 and the various elements formed therein. A portion of thethermal oxide layer associated with the active region 14 is subsequentlyremoved to form a Schottky recess in which the Schottky layer 24 isformed.

Once the Schottky recess is formed, the Schottky layer 24 is formed overthe portion of drift layer 22 that was exposed by the Schottky recess.The thickness of the Schottky layer 24 will vary based on desired devicecharacteristics and the metal used to form the Schottky layer 24, butwill generally be between about 100 and 4500 angstroms. For thereferenced 650V device, a Schottky layer 24 formed of tantalum (Ta) maybe between about 500 and 1500 angstroms; a Schottky layer 24 ofpolysilicon may be between about 1000 and 5000 angstroms; and a Schottkylayer 24 formed of titanium (Ti) may be between about 500 and 2500angstroms. As noted above, tantalum (Ta) is associated with a lowbarrier height, especially when used in combination with SiC to form aSchottky junction. Tantalum is also very stable against SiC. Dependingon the metal used for the Schottky layer 24 and the to-be-formed anodecontact 26, one or more barrier layers 28 may be formed over theSchottky layer 24. The barrier layer 28 may be formed of titaniumtungsten alloy (TiW), titanium nickel alloy (TiN), tantalum (Ta), andany other suitable material, and may be between about 100 and 1000angstroms thick in select embodiments. The barrier layer 28 helpsprevent diffusion between the metals used to form the Schottky layer 24and the to-be-formed anode contact 26. Notably, the barrier layer 28 isnot used in certain embodiments where the Schottky layer 24 is tantalum(Ta) and the to-be-formed anode contact 26 is formed from aluminum (Al).The barrier layer 28 is generally beneficial in embodiments where theSchottky layer 24 is titanium (Ti) and the to-be-formed anode contact 26is formed from aluminum (Al).

The anode contact 26 is formed over the Schottky layer 24, or ifpresent, the barrier layer 28. The anode contact 26 is generallyrelatively thick, formed from a metal, and acts as a bond pad for theanode of the Schottky diode 10. The anode contact 26 may be formed fromaluminum (Al), gold (Au), Silver (Ag), and the like.

As noted above, at the end of fabrication, the substrate 12 may besubstantially thinned by removing a bottom portion of the substrate 12though a grinding, etching, or like process. For the 650V referenceSchottky diode 10, the substrate 12 may be thinned to a thicknessbetween about 50 and 200 microns in a first embodiment. Thinning thesubstrate 12 or otherwise employing a thin substrate 12 reduces theoverall electrical and thermal resistance between the anode and cathodeof the Schottky diode 10 and allows the device to handle higher currentdensities without overheating.

The cathode ohmic layer 20 is formed on the bottom of the thinnedsubstrate 12 with an ohmic metal, such as nickel (Ni), nickel silicide(NiSi), and nickel aluminide (NiAl). Once the cathode ohmic layer 20 isformed and annealed, the cathode contact 18 is formed over the cathodeohmic layer 20 to provide a solder or like interface for the Schottkydiode 10.

With the concepts disclosed herein, very high performance Schottkydiodes 10 may be designed for various applications that require variousoperation parameters. The current density associated with DC forwardbiased currents may exceed 500 amperes/cm² in certain embodiments. Invarious emobiments, leakage current density has been held below about 5milliamperes at temperatures above about 125, 150, and 175 Celsius, aswell as throughout a range of 150 to 200 Celsius.

Those skilled in the art will recognize that the concepts discussedabove may be implemented in designs that differ from those specificallydisclosed herein. For example and with reference to FIG. 8, the Schottkydiode 10 may employ an edge termination region 16 that is recessedrelative to the active region. For example, the top of the drift layer22 in the active region 14 is higher than the top of the drift layer 22in the edge termination region 16. In the embodiment illustrated,substantially all of the buffer region 38 extends above the top surfaceof those portions of the drift layer 22, which reside in the terminationregion 16. Those skilled in the art will recognize further improvementsand modifications to the embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A Schottky diode comprising: a drift layerpredominantly doped with a doping material of a first conductivity typeand having a first surface associated with an active region; a Schottkylayer over the active region of the first surface to form a Schottkyjunction; a plurality of junction barrier elements formed in the driftlayer below the Schottky junction; and a plurality of central implantsformed in the drift layer below the Schottky junction.
 2. The Schottkydiode of claim 1 wherein at least one of the plurality of centralimplants is provided between and spaced apart from adjacent pairs of theplurality of junction barrier elements.
 3. The Schottky diode of claim 2wherein each of the plurality of central implants has no greater thanabout one-half of a depth of each of the plurality of junction barrierelements.
 4. The Schottky diode of claim 2 wherein each of the pluralityof junction barrier elements are at least four times deeper than each ofthe plurality of central implants.
 5. The Schottky diode of claim 3wherein each of the plurality of junction barrier elements and each ofthe plurality of central implants are doped at approximately the samedoping concentration with a doping material of a second conductivitytype.
 6. The Schottky diode of claim 3 wherein each of the plurality ofjunction barrier elements and each of the plurality of central implantsare doped at a concentration of at least 1×10¹⁸ cm⁻³.
 7. The Schottkydiode of claim 3 wherein the first surface of the drift layer comprisesa plurality of junction barrier element recesses in the active regionsuch that at least certain junction barrier elements of the plurality ofjunction barrier elements are doped regions that extend into the driftlayer about corresponding ones of the plurality of junction barrierelement recesses, and the doped regions are doped with a doping materialof a second conductivity type, which is opposite the first conductivitytype.
 8. The Schottky diode of claim 3 further comprising a bufferregion in a top portion of the drift layer wherein the buffer region isdoped with the doping material of the first conductivity type at ahigher concentration than a remaining lower portion of the drift layerand both the plurality of central implants and the plurality of junctionbarrier elements reside in the buffer region.
 9. The Schottky diode ofclaim 3 wherein the Schottky layer is formed from a low barrier heightcapable metal.
 10. The Schottky diode of claim 9 wherein the Schottkyjunction has a barrier height of less than 0.9 electron volts.
 11. TheSchottky diode of claim 9 wherein the low barrier height capable metalof the Schottky layer comprises tantalum.
 12. The Schottky diode ofclaim 11 wherein the drift layer comprises silicon carbide.
 13. TheSchottky diode of claim 9 wherein the low barrier height capable metalof the Schottky layer consists essentially of tantalum.
 14. The Schottkydiode of claim 9 wherein the low barrier height capable metal of theSchottky layer comprises at least one of a group consisting of titanium,chromium, polysilicon, and aluminum.
 15. The Schottky diode of claim 3wherein the drift layer is formed over a thinned substrate that isthinned after the drift layer is formed and a cathode contact is formedover a bottom surface of the thinned substrate.
 16. The Schottky diodeof claim 3 wherein the active region is substantially surrounded by anedge termination region, which comprises an edge termination structureformed in the drift layer.
 17. The Schottky diode of claim 16 wherein aportion of the drift layer for the edge termination region is recessedrelative to the first surface of the active region.
 18. The Schottkydiode of claim 1 wherein the first surface of the drift layer comprisesa plurality of junction barrier element recesses in the active regionsuch that at least certain junction barrier elements of the plurality ofjunction barrier elements are doped regions that extend into the driftlayer about corresponding ones of the plurality of junction barrierelement recesses, and the doped regions are doped with a doping materialof a second conductivity type, which is opposite the first conductivitytype.
 19. The Schottky diode of claim 18 further comprising a bufferregion in a top portion of the drift layer, wherein the buffer region isdoped with the doping material of the first conductivity type at ahigher concentration than a remaining lower portion of the drift layerand both the plurality of central implants and the plurality of junctionbarrier elements reside in the buffer region.
 20. The Schottky diode ofclaim 1 further comprising a buffer region in a top portion of the driftlayer wherein the buffer region is doped with the doping material of thefirst conductivity type at a higher concentration than a remaining lowerportion of the drift layer and both the plurality of central implantsand the plurality of junction barrier elements reside in the bufferregion.
 21. The Schottky diode of claim 1 wherein the Schottky layer isformed from a low barrier height capable metal.
 22. The Schottky diodeof claim 21 wherein the Schottky junction has a barrier height of lessthan 0.9 electron volts.
 23. The Schottky diode of claim 21 wherein thelow barrier height capable metal of the Schottky layer comprisestantalum.
 24. The Schottky diode of claim 23 wherein the drift layercomprises silicon carbide.
 25. The Schottky diode of claim 21 whereinthe low barrier height capable metal of the Schottky layer consistsessentially of tantalum.
 26. The Schottky diode of claim 21 wherein thelow barrier height capable metal of the Schottky layer comprises atleast one of a group consisting of titanium, chromium, polysilicon, andaluminum.
 27. The Schottky diode of claim 1 wherein a leakage currentdensity is below about 5 milliamperes at temperatures above about 125Celsius.
 28. The Schottky diode of claim 1 wherein a leakage currentdensity is below about 5 milliamperes at temperatures above about 150Celsius.
 29. A Schottky diode comprising: a drift layer predominantlydoped with a doping material of a first conductivity type and having afirst surface associated with an active region; a Schottky layer of alow barrier height capable metal provided over the active region of thefirst surface to form a Schottky junction; a plurality of junctionbarrier elements formed in the drift layer below the Schottky junction;and a plurality of central implants formed in the drift layer below theSchottky junction, wherein at least one of the plurality of centralimplants is provided between and spaced apart from adjacent pairs of theplurality of junction barrier elements; each of the plurality of centralimplants has no greater than about one-half of a depth of each of theplurality of junction barrier elements; the first surface of the driftlayer comprises a plurality of junction barrier element recesses in theactive region such that at least certain junction barrier elements ofthe plurality of junction barrier elements are doped regions that extendinto the drift layer about corresponding ones of the plurality ofjunction barrier element recesses, and the doped regions are doped witha doping material of a second conductivity type, which is opposite thefirst conductivity type.
 30. The Schottky diode of claim 29 furthercomprising a buffer region in a top portion of the drift layer whereinthe buffer region is doped with the doping material of the firstconductivity type at a higher concentration than a remaining lowerportion of the drift layer and both the plurality of central implantsand the plurality of junction barrier elements reside in the bufferregion.
 31. The Schottky diode of claim 30 wherein the Schottky junctionhas a barrier height of less than 0.9 electron volts.
 32. The Schottkydiode of claim 31 wherein the low barrier height capable metal of theSchottky layer comprises tantalum.
 33. The Schottky diode of claim 32wherein the drift layer comprises silicon carbide.
 34. The Schottkydiode of claim 29 wherein the low barrier height capable metal of theSchottky layer consists essentially of tantalum.
 35. The Schottky diodeof claim 29 wherein the low barrier height capable metal of the Schottkylayer comprises at least one of a group consisting of titanium,chromium, polysilicon, and aluminum.
 36. The Schottky diode of claim 29wherein a leakage current density is below about 5 milliamperes attemperatures above about 125 Celsius.
 37. The Schottky diode of claim 29wherein a leakage current density is below about 5 milliamperes attemperatures above about 150 Celsius.